A biaxially-oriented film of polyethylene terephthalate (PET) has heretofore been often used as a non-magnetic support (hereinafter also referred to as "base") for a magnetic recording medium because of its good surface flatness and mechanical properties and excellent abrasion resistance and running properties in the form of magnetic recording medium. In recent years, in the field of video tape comprising polyester as a base, demand for higher recording density and prolonged recording time is increased. This requires that the base have excellent smoothness and slip properties, a smaller thickness, a higher strength and excellent running properties.
In order to solve these problems, magnetic tape manufacturers employ a PET base which is strengthened both longitudinal and crosswise directions. However, such a PET base is disadvantageous in that as its thickness decreases, it lacks strength. Such a PET base causes deformation or damage on tape edge in a tape running system, remarkably impairing the recording and reproduction properties of the tape.
As approaches for providing a strengthened thin magnetic tape there are proposed a technique using polyaramide in JP-A-62-234233 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and a technique using polyethylene naphthalate (PEN) in EP-229346. These bases attain a high strength because these materials have a strong in-plane orientability. At the same time, this strong in-plane orientability causes a remarkably reduced tear propagation resistance. Therefore, the magnetic recording material is liable to break at coating, calendering and slitting steps, causing a drastic yield drop. Even after formed into a tape, the magnetic recording material can be extremely easily broken when damaged even slightly due to some trouble in the running system, raising great problems in product quality. JP-A-63-197643 and JP-A-63-212549 propose a technique using a composite polyester. However, such a technique is disadvantageous in that if a proper lamination ratio is not selected, the strong in-plane orientability causes a remarkable drop of tear propagation resistance or a lack of strength. Accordingly, the magnetic recording material is liable to break at coating, calendering and slitting steps, causing a drastic yield drop or deformation or damage on tape edge in a tape running system and hence remarkably impairing the recording and reproduction properties of the magnetic recording medium.
In some detail, JP-A-63-197643 proposes a three-layer composite film composed of a biaxially-oriented PET film as a core layer and a biaxially-oriented PEN film provided on the sides thereof, the thickness of said outer layers being from 1/40 to 1/5 of the total thickness of the film.
The foregoing structure is an attempt to inhibit the precipitation of an oligomer crystal that causes dropout. However, the foregoing structure is disadvantageous in that it lacks longitudinal (MD) and crosswise (TD) strength in an ordinary balance type, causing edge deformation that results in a drastic output drop.
PEN is known as a material having an extremely high strength and thus is still a material suitable for the prolongation of recording time, enhancement of recording density and miniaturization. In other words, a PEN material which can be effectively used singly as a base and a method for using such a PEN material have been desired.
However, even if a high strength base such as PEN is used, the reduction of the total thickness of the magnetic tape gives a great contribution to the drop of the tape strength. This lowers head contact pressure, deteriorating RF output and RF flatness and worsening head stain. It has thus been desired to solve these problems. However, magnetic recording media having a base satisfying these requirements have never been obtained.
The use of a high strength base is also disadvantageous in that the slittability of the magnetic tape is impaired, causing the tape to rise at its edge during slitting.